1. Field of the Invention
The subject invention relates to a method and apparatus for coating prosthetic components to achieve a uniform shiny smooth coating that will exhibit exceptional wear when placed in articular or sliding bearing engagement with another prosthetic component.
2. Description of the Related Art
Orthopedic prosthetic joints are used to replace at least portions of a diseased or damaged natural joint between at least two bones of a patient. A total joint replacement typically involves accessing the diseased or damaged joint and resecting opposed ends of the bone that form the joint. Metallic components then are affixed to the resected ends of the bones.
Metal on metal sliding or articular contact is known to produce molecular or chemical reactions that generate wear debris and degrade the performance of the prosthetic joint. As a result, the typical prior art prosthetic joint is designed to have metallic components of the prior art prosthetic joint articulate or slide against a non-metallic component. For example, plastic bearings may be incorporated into a joint for sliding and/or articular bearing engagement with the metallic components of the joint. The plastic bearing should be hard, wear resistant and chemically inert in the presence of the biological fluids. Many prior art prosthetic joints employ bearings made of an ultra high molecular weight polyethylene (UHMWPe). The UHMWPe bearing will not cause the chemical reactions that occur when metallic components slide against one another. However, UHMWPe bearings can wear if the metallic part that slides against the bearing is rough. In view of the above, it is important to provide very smooth surfaces for articulation or sliding against a UHMWPe bearing.
Many prosthetic joints employ metallic components formed from alloys of titanium or cobalt-chromium. Cobalt chromium alloys are very hard and strong and exhibit appropriate flexure. However, cobalt chromium also is very expensive and many patients exhibit sensitivity to cobalt-chromium alloys. Titanium alloys, such as titanium aluminum vanadium alloys also exhibit appropriate strength and flexure. Additionally, patients are less likely to exhibit sensitivity to titanium alloys, and titanium alloys are less expensive than cobalt-chromium alloys. However, titanium alloys generally are not as hard as cobalt chromium alloys. As a result, articulating surfaces of prosthetic components formed from titanium alloys typically are provided with a thin ceramic coating. For example, the articular surfaces of a titanium alloy prosthetic component may be provided with a titanium nitride (TiN) coating.
Ceramic coatings typically have been applied to metallic alloy substrates by known physical vapor deposition processes. This technology is widely used, for example, in the machine tool art to enhance the life of cutting tools. Prior art physical vapor deposition technology-typically applies the coating to a substrate in a vacuum coating chamber. A charge is applied to the substrate to be coated in the chamber, while an opposite charge is applied to the material to be coated onto the substrate. An arc is struck in the chamber, and the substrate to be coated is subjected to high energy ion bombardment. A gas then is introduced into the chamber. The gas reacts with the ions of the cathode and produces an ionic deposition of a highly-adherent ceramic coating onto the substrate.
FIG. 1 hereto schematically illustrates the prior art vacuum coating chamber for applying a TiN coating to a titanium alloy prosthetic component. In particular, FIG. 1 shows a vacuum chamber 10 having a plurality of surface-mountable femoral components 12 for replacement hips. Each femoral component 12 includes a mounting stem 14 that will be mounted in a mounting aperture formed in the resected proximal end of the femur. The femoral component 12 further includes a head 16 having a convex bearing surface 18 and an opposed concave surface (not shown) for nesting tightly over the resected proximal end of the femur. The convex bearing surface 18 of the femoral component 12 will be in articular bearing engagement with a plastic bearing liner of the acetabular component of the hip prosthesis. The plastic bearing liner typically will be formed from UHMWPe. The prior art vacuum chamber 10 is employed to impart a TiN coating to the convex articular bearing surface 18 of the prior art femoral component 12. As noted above, the coating must be hard to prevent wear of the coating and must be smooth to prevent wear of the UHMWPe bearing against which the femoral component 12 will articulate.
In an effort to achieve uniform coating, the prior art chamber includes a fixture 20 that extends into the prior art chamber 10. The prior art fixture 20 is in a substantially central position within the prior art chamber 10 and is constructed to permit the femoral components 12 to be mounted thereon such that the convex articular bearing surfaces 18 of the femoral components 12 face outwardly and away from fixture 20. The prior art fixture 20 is mounted to a power supply 22 disposed externally of the prior art chamber 10. The power supply 22 imparts a charge to the fixture and to the femoral components 12 in the prior art chamber 10. The power supply 22 further functions to rotate the prior art fixture 20 within the prior art chamber 10 in an effort to achieve uniform coating on the convex bearing surfaces 18 of the prosthetic components 12.
The prior art chamber 10 further includes titanium evaporator 24 mounted in the walls of the chamber 10 and facing the prosthetic components 12. A charge is applied to the evaporators 24 that is opposite to the charge applied to the fixture 20. An arc then is struck in the chamber 10 and the convex articular bearing surfaces 18 of the prosthetic components 12 are subject to an ionic bombardment. Nitrogen gas is introduced into the chamber 10. The gas reacts with the ions and produces an ionic deposition of a highly-adherent ceramic TiN coating onto the articular bearing surfaces 18 of the prosthetic components 12.
Conventional wisdom, as practiced in the prior art, is to employ the prior art fixture 20 with the articular bearing surfaces 18 of the prosthetic components 12 facing the evaporators 24 in an effort to achieve optimum coating.
It recently was observed by the applicant herein that parts produced by the above-described prior art chamber 10 were slightly dull at certain locations and shinier in other locations. Analysis was conducted to determine the reason for the different appearances at different locations on the bearing surfaces 18 of the prosthetic components 12. It was determined that the improperly coated prosthetic components were not properly rotated in the prior art chamber 10, and thus one side was not directly exposed to the ion bombardment. Dull regions on the improperly coated prosthetic components 12 were determined to be attributable to very small droplets formed on the prosthetic components 12. The droplets form an asperity on the bearing surface 18 which would increase wear against UHMWPe bearings.
Accordingly, it is an object of the subject invention to provide a coating apparatus and process that will limit asperity and thereby provide a bearing surface with excellent wear characteristics.
The above-described malfunctioning apparatus demonstrated that one side of the prosthetic component in the prior art chamber with the malfunctioning fixture had a high quality coating, while the opposed side had an inferior coating. In an effort to determine which sides of the prosthetic components achieved the better coating, a chamber was developed which fixtured the prosthetic components so that the surface that required a smooth hard coating would define the xe2x80x9cdark sidexe2x80x9d of the chamber. Thus, the fixtures were arranged so that the convex articular bearing surfaces of the prosthetic components faced away from the evaporators and toward the center of the chamber. Each individual component""s surface to be coated, or xe2x80x9cdark sidexe2x80x9d, faces away from its most proximal evaporator so as to be shielded from the direct straight line-of-sight path from the cathode to the surface. Contrary to conventional wisdom and contrary to the practice in the prior art, it was determined by the applicant that prosthetic components that have been fixtured with the surface to be coated facing away from the evaporator in this fashion, and hence defining the xe2x80x9cdark sidexe2x80x9d, achieves a uniformly shiny coated surface. Furthermore, it has been determined that the coating achieved by this process and in this chamber is substantially free of the small droplets that are provided with prior art coating processes and chambers and exhibit superior wear characteristics particularly against UHMWPe.
It is believed that the physical vapor deposition process produces both single ions and double or multiple ions within the chamber. The multiple ions, of course, have greater mass and will produce the above-described small droplets and corresponding unevenness on the surface of the prosthetic component to be coated. When the side to be coated defines the xe2x80x9cdark sidexe2x80x9d and faces away from the most proximal ion source, the momentum attributable to the higher mass multiple ions cause those heavier ions to travel substantially linearly past the xe2x80x9cdark sidexe2x80x9d surface to be coated. However, the lower mass single ions are more likely to travel through a curved path and be attracted by the oppositely charged prosthetic components facing away from the most proximal titanium anode. Thus, only single ions are deposited on the xe2x80x9cdark sidexe2x80x9d surface that requires the smooth coating. Although this xe2x80x9cdark sidexe2x80x9d coating process takes longer, the results have proven to be superior with a very smooth coating substantially free of droplets that had been present in the prior art. As a result, prosthetic components formed by the subject apparatus and accordance with the process described herein do not require extensive polishing and achieve superior wear when in articular bearing engagement with a UHMWPe bearing.